Radiation emitting device

ABSTRACT

A radiation emitting device comprising a first electrode, which emits first charge carriers having a first charge during operation, a first charge carrier transporting layer, which comprises a fluorescent substance, a second charge carrier transporting layer, which contains a phosphorescent substance, and a second electrode, which emits second charge carriers having a second charge during operation, wherein during operation the second charge carrier transporting layer is largely free of first charge carriers.

RELATED APPLICATIONS

This patent application claims the priorities of German patentapplication 10 2007 037 097.2 filed Aug. 7, 2007 and of German patentapplication 10 2007 053 396.0 filed Nov. 1, 2007, the disclosurecontents of both of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention is related to a radiation emitting device.

BACKGROUND OF THE INVENTION

A widespread problem of radiation emitting devices is the low yield ofemissive radiation in relation to the voltage applied to the devices.

SUMMARY OF THE INVENTION

One object of the invention is to provide a radiation emitting devicewhich has an increased yield of emitted radiation.

This and other objects are attained in accordance with one aspect of theinvention directed to a radiation emitting device comprising a firstelectrode, which emits first charge carriers having a first chargeduring operation, a first charge carrier transporting layer, whichcomprises a fluorescent substance and is arranged on the firstelectrode, a second charge carrier transporting layer, which contains aphosphorescent substance and is arranged on the first charge carriertransporting layer, a second electrode, which emits second chargecarriers having a second charge during operation and is arranged on thesecond charge carrier transporting layer, wherein during operation ofthe device the second charge carrier transporting layer is largely freeof first charge carriers.

In a radiation emitting device having a layer sequence of this type, thesecond charge carrier transporting layer is largely free of chargecarriers having the first charge which are emitted by the firstelectrode during operation.

In this case, “largely free” should be understood to mean that thecharge carriers having the first charge in the second charge carriertransporting layer, during operation of the device, have a proportion ofthe charge transport in said layer which makes up at most a proportionof 0.1% of the total charge transport in said layer. This can forexample mean, in particular, that the difference between the chargetransport mediated by the first charge carriers and the charge transportmediated by the second charge carriers is at least three powers of ten.The consequence of this is that in said layer during operation of thedevice almost only second charge carriers migrate and thereforepractically no charge carriers having the first and the second chargecan meet one another and recombine. As a result, in said layer, in whicha phosphorescent substance is situated, no energy that could excite thephosphorescent substance into an electronically excited state can bereleased by recombination of the first and second charge carriers.Consequently, the energy for the excitation of the phosphorescentsubstance has to be transferred from the first charge carriertransporting layer into the second charge carrier transporting layer.

The second charge carrier transporting layer can be unipolar and/orblock charge carriers having the first charge. Consequently, during theoperation of the device, the second charge carrier transporting layerthen transports only second charge carriers but does not transport firstcharge carriers, or transports said first charge carriers only to asubordinate extent, with the consequence that the second charge carriertransporting layer is largely free of first charge carriers during theoperation of the device.

Consequently, one embodiment of the invention comprises at least twoelectrodes between which at least two charge carrier transporting layersare arranged, at least one layer of which transports only chargecarriers having the second charge, and also at least one further layerin which charge carriers having both the first and second charges aretransported.

If charge carriers having the first charge and charge carriers havingthe second charge, which is charged oppositely to the first charge, meetone another in the first charge carrier transporting layer, then theycan recombine to form excited electronic states (excitons). The excitonis a two-particle state comprising excited negatively charged electronand positively charged charge carrier, the so-called “hole” or defectelectron. If a molecule is in the electronically excited state, thenthere are a number of possibilities for emitting the energy again viadifferent relaxation processes. In the case of thermal relaxation, theenergy is emitted without radiation as thermal energy. A furtherpossibility is energy emission in the form of radiation, for examplevisible radiation in the range of approximately 400 to 800 nm, andradiation in the UV range and infrared IR range. A distinction is madehere essentially between two cases: fluorescence and phosphorescence.Fluorescence is understood here to mean the emission of radiation byrelaxation from an electronically excited singlet state in which theelectrons have paired electron spins in the excited state. By contrast,phosphorescence is understood to mean a relaxation process in which therelaxation takes place from an electronically excited triplet state inwhich the electrons have parallel electron spins in the excited state.

An excited fluorescent substance can thus emit radiation in the form offluorescence. The transition from a singlet state to a triplet state isactually forbidden on account of the associated spin reversal, since thespin reversal contravenes the quantum mechanical conservation of thespin angular momentum. Consequently, a spin reversal is only possible ifthe change in the spin angular momentum is compensated for by the changein another angular momentum, which usually takes place by way of theelectronic orbital angular momentum.

The recombination of electron and “hole” gives rise to an excitedelectronic state, an exciton. An exciton is a two-particle state whosespin half particles, electron and hole, can combine to give a total spin0, a singlet exciton, or a total spin 1, a triplet exciton. On accountof spin statistics, singlet and triplet excitons form in the ratio of1:3.

Fluorescent or phosphorescent substances can be converted to an excitedstate by the energy of said excitons. Relaxation is then in turnpossible from said state, the released energy being emitted in the formof visible radiation, for example, in the course of said relaxation.

In the second charge carrier transporting layer, owing to the fact thatcharge carriers having the first charge are largely absent, it ispractically impossible for charge carriers having the first and thesecond charge to meet one another, and, consequently, it is also notpossible for the two charge carriers to recombine to form excitons.Consequently, the phosphorescent substance present in said second chargecarrier transporting layer is put into an excited state by the energytransported by the excitons, preferably the triplet excitons of thefirst charge carrier transporting layer by means of energy transfermechanisms into the second charge carrier transporting layer.

It is desirable for a device to obtain a highest possible radiationyield, and to have the possibility of being able to cover a widestpossible radiation spectrum. A good radiation yield means a highestpossible quotient of radiation yield obtained in relation to the voltageapplied to the first and second electrode. For this purpose, it isdesirable not only to convert the energy into radiation energy which ispresent in an excited electron singlet state, but also to use the energyfor radiation generation which is present in excited electronic tripletstates. The latter makes up 75% on account of spin statistics, since asecond electron in an excited electronic state can assume threedifferent orientation possibilities with respect to the spin of a firstelectron which lead to a triplet state, whereas only one orientationpossibility leads to the spin pairing and thus to the singlet state. Theattempt, in addition to the singlet emission, also to convert a largestpossible part of the energy still present into triplet emission is alsoreferred to as “triplet harvesting”. In this case, the phosphorescentsubstance serves to take up the excited electronic states present in thetriplet excitons of the fluorescent substance by means of energytransfer mechanisms. By means of radiative relaxation from said tripletexcitons of the phosphorescent substance, the energy present in thetriplet states is then used with respect to the radiation yield of theradiation emitting device. If the phosphorescent substance were notpresent in the radiation emitting device, then the triplet excitons ofthe fluorescent substance would relax by means of radiationlessquenching processes, with the consequence that the excited electronicstates present in the triplet excitons of the fluorescent substance makeno contribution or only a subordinate contribution to the radiation ofthe radiation emitting device.

Further layers can be present between the electrodes and the two chargecarrier transporting layers, and also the two charge carriertransporting layers themselves.

Thus, in one embodiment of the invention, a unipolar charge carriertransporting layer, which blocks the charge carriers having the firstcharge or can transport only charge carriers having the second charge,is arranged between the first charge carrier transporting layer and thesecond charge transporting layer. Consequently, penetration of chargecarriers having the first charge into the second charge carriertransporting layer can be prevented or reduced, with the consequencethat the second charge carrier transporting layer is largely free ofcharge carriers having the first charge during the operation of thedevice. In this case, the energy levels of the matrix of the secondcharge carrier transporting layer do not have to be selected in regardto this circumstance.

In accordance with a further embodiment according to the invention,charge carrier transport paths run through the radiation emitting deviceduring the operation of the device. In connection with this invention,charge carrier transport paths are understood to mean the path that canbe taken by the charge carriers during operation perpendicular to thelayer course, that is to say through the layers coming from oneelectrode in the direction of the electrode having the opposite charge.In this case, for the charge carriers having the first charge, a chargecarrier transport path occurs which is predefined by that region of thelayer sequence of the device which is delimited by the first electrodeand the first charge carrier transporting layer. For the charge carriershaving the second charge, a charge carrier transport path results which,proceeding from the second electrode, extends at least through thesecond charge carrier transporting layer into the first charge carriertransporting layer. Consequently, during the operation of the device,the charge carrier transport path for the first charge carriers isshorter than the charge carrier transport path for the second chargecarriers.

During the operation of the device, the corresponding charge carrierscan pass through layers which lie between the electrodes mentioned andthe charge carrier transporting layers to be reached in each case. Thus,by way of example, a charge carrier having the first charge can passthrough an exciton blocking layer situated between the first electrodeand the first charge carrier transporting layer, which correspondinglylengthens the charge carrier transport path in comparison with a layersequence in which the exciton blocking layer is not present. If theexciton blocking layer lies between the second electrode and the secondcharge carrier transporting layer, then the charge carriers having thesecond charge can pass through it during the operation of the device,whereby the charge carrier transport path of the charge carriers havingthe second charge would be correspondingly lengthened.

In a further embodiment of the invention, the radiation emitting deviceis for example an organic light emitting diode (OLED) comprising one ormore organic layers or layers containing organic materials. The organicsubstances can be for example the material of the first charge carriertransporting layer and/or second charge carrier transporting layer, andalso the phosphorescent and/or fluorescent substance. Furthermore, thefluorescent and/or phosphorescent substance can also have charge carriertransporting properties and, consequently, the respective charge carriertransporting layer can predominantly consist of said substances orcomprise them as basic components. However, the fluorescent and/orphosphorescent substance, irrespective of whether said substance is oforganic or inorganic nature, can also be present as a dopant in a matrixmaterial. In this case, the dopant itself can contribute to the chargetransport, but this is not obligatory. The organic substances can bepolymeric electroluminescent substances or non-polymeric substances,so-called “small molecules”. In this case, electroluminescence isunderstood to mean the property that a substance can be excited at leastpartly to emit radiation, e.g. light, by applying an electrical voltage.

The OLED can have a substrate on the side of the first electrode andalso on the side of the second electrode, on which substrate thefunctional layer stack comprising the charge carrier transporting layerscan be arranged.

In a further embodiment of a device according to the invention, thefirst electrode can be an anode and also a cathode. The same applies tothe second electrode as well, wherein the second electrode has theopposite polarity to the first electrode. That is to say that if thefirst electrode is a cathode, then the second electrode is an anode.Likewise, it is possible for the first electrode to be an anode, and thesecond electrode a cathode.

The charge carriers having the first charge can therefore be negativecharge carriers, electrons, if the first electrode is a cathode, andalso positive charge carriers, so-called “holes”, if the first electrodeis an anode. The charge carriers having the second charge have thecorresponding opposite polarity to the charge carriers having the firstcharge. That is to say that if the second electrode is an anode, thenthe charge carriers having the second charge are holes. However, theopposite case where the charge carriers having the first charge areholes if the first electrode is an anode, and the charge carriers havingthe second charge are electrons, is likewise possible as well.

If an embodiment according to the invention comprises an anode as firstelectrode, then the following layers are a hole transporting andelectron transporting layer as first charge carrier transporting layer,an electron transporting layer as second charge carrier transportinglayer and a cathode as second electrode.

If an embodiment according to the invention comprises a cathode as firstelectrode, then the following layers are a hole transporting andelectron transporting layer as first charge carrier transporting layer,a hole transporting layer as second charge carrier transporting layerand an anode as second electrode.

In a further embodiment according to the invention, the radiationemitting device can comprise a second electrode as anode and a firstelectrode as cathode. The second electrode, the anode, is followed by ahole inducting layer (HIL), a second charge carrier transporting layerin the form of a hole transporting layer (HTL) and a first chargecarrier transporting layer in the form of an electron transporting layer(ETL). When the forward potential is applied, the anode injects positivecharge carriers, so-called holes, and the cathode injects electrons intothe organic layers. The injected holes and electrons migraterespectively to the opposite charged electrodes. Recombination of holeswith electrons gives rise to so-called excited electronic states, theexcitons.

Fluorescent or phosphorescent substances can be excited by the energy ofsaid excitons, said substances then emitting radiation. Two emissionroutes are available for the emission of visible radiation. Firstlyrelaxation from an excited singlet state, and secondly relaxation froman excited triplet state. A triplet state is distinguished by the factthat the electrons of the excited electronic state have anequidirectional spin, whereas in a singlet state the electrons of theexcited electronic state have an opposite spin (the spins are “paired”).In addition, however, the excited states can also emit their energy bymeans of radiationless quenching processes, or else the emittedradiation does not lie in the visible range. In connection with thisinvention, the term “radiation” is used under the meaning ofelectromagnetic radiation in the range of IR to UV in each caseinclusive. In order to obtain an emission in the visible range, thedifferent layers of an OLED are doped with corresponding substances, orproduced from said substances.

In the case of the hole transporting layer (HTL), both the position ofthe highest occupied molecular orbital (HOMO) and the separation betweenHOMO and the lowest unoccupied molecular orbital (LUMO) should beadapted to the molecular orbitals HOMO/LUMO of the other layers andmaterials of the OLED. The HOMO level must be low enough to be able toinject the holes from the anode. On the other hand, however, it ispermitted to be only low enough that the required energy barrier withrespect to the HOMO of the electron transporting layer (ETL) is not toogreat, such that the holes can migrate in the electron transportinglayer in order to be able to recombine with the electrons there. Bycontrast, if no recombination is intended to take place in the electrontransporting layer, that is to say no holes are intended to pass intosaid layer, then the HOMO of the HTL must be chosen to becorrespondingly lower than that of the ETL. This gives rise to a holebarrier, that is to say a hole blocking layer, at this location of theOLED.

The HOMO-LUMO separation of the HTL, and also of other layers, should bechosen to be sufficiently large, larger than in the case of the emissivedopant, in order that the emitted radiation is not immediately absorbedagain in the layer itself. The energy difference between LUMO-HTL andLUMO-ETL must be chosen according to whether or not electrons areintended to pass into the HTL. If no electrons are intended to pass intothe HTL, that is to say that the latter is intended to serve as anelectron barrier, then the LUMO of the HTL must be at a higher energeticlevel than the LUMO of the ETL. The LUMO of an electron blocking layershould preferably be at least 300 meV above that of the electrontransporting layer. By contrast, if the recombination is intended to beable to take place in the HTL and the electrons are intended to be ableto migrate into the HTL, then the energy difference between LUMO-HTL andLUMO-ETL must be kept small. The ETL should furthermore have a LUMO thatis low enough in order that the electrons can be injected from thecathode into said LUMO. The material of the cathode should becoordinated with this, if appropriate. If the OLED also comprises a holeinjecting layer (HIL), then the energy levels should be adapted to thoseof the anode and of the HTL, that is to say that they must lie betweenthe two, which reduces the work function of the holes from the anodeinto the succeeding layer, here the HIL, and thus facilitates the holetransition from the anode to the subsequent layer. The HOMO of the HILshould be not more than 700 meV above that of the anode, preferably notmore than 500 meV.

If a layer has a hole blocking function, by contrast, then its HOMO mustlie below the HOMO of the layer from which the holes are intended to bekept away. In this case, the HOMO of the blocking layer should lie atleast 300 meV, but more expediently 700 meV, below that of thesubsequent layer.

One embodiment of an OLED according to the invention can also comprisean exciton blocking layer. Such a layer would preferably be locatedbetween the second charge carrier transporting layer and the secondelectrode. The energy levels of said layer must likewise be coordinatedwith those of the adjacent layers. In this case, the energy difference(ΔE) between HOMO and LUMO is of importance in the case of the excitonblocking layer. Said energy difference defines the energy of theexciton. Consequently, an exciton cannot diffuse, or can diffuse only toa subordinate extent, into a layer whose ΔE is 100 meV, or more, greaterthan the energy of the exciton, or the ΔE of the layer in which theexciton was formed. Furthermore, an important factor is whether saidlayer is intended to conduct electrons or holes. The energy level of theLUMO and HOMO should be chosen accordingly. If the electrons areintended to be blocked, for example, then the LUMO of the blocking layermust be higher than that of the layer in which the electrons aretransported toward the blocking layer.

Irrespective of the presence of the first exciton blocking layer, infurther devices a second exciton blocking layer can also be present forexample between the first charge carrier transporting layer and thelayer which injects the charge carriers having the first charge into thefirst charge carrier transporting layer. Said exciton blocking layerwould prevent diffusion of the excitons formed in the first chargecarrier transporting layer into the injecting layer.

However, if an exciton blocking layer is positioned before a cathode,for example, then it is thereby possible to prevent the quenching of theexcitons and the associated conversion into radiationless energy.

In further embodiments of this invention it is possible to coordinatethe energy levels of the individual layers with one another. For thispurpose, either the matrix materials of the first and second chargecarrier transporting layers or the dopants which are introduced into thematrix layers of said layers must have corresponding energy levels. TheHOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest UnoccupiedMolecular Orbital) levels of the substances can be determined in variousways. One common method for determining the HOMO level is ultravioletphotoelectron spectroscopy (UPS). Inverse photoemission spectroscopy(IPS), for example, is suitable for determining the LUMO level. Theenergy difference (ΔE) between HOMO and LUMO can be calculated directlyby comparing absorption and emission spectrums. Reference should be madeto the corresponding textbooks for the mode of operation of thetechniques mentioned above.

In order to reduce or avoid quenching of the phosphorescence dopants asphosphorescent substance by the matrix substance into which it wasintroduced, the energy level of the dopant from which phosphorescencetakes place (T₁ level) should lie between the T₁ level of the matrixsubstance. That is because if the T₁ level of the matrix substance liesbelow that of the dopant, then a radiationless triplet exciton transferfrom the dopant to the matrix can occur. On account of their low T₁levels, therefore, polymeric compounds having good charge carriertransport properties are suitable only to a limited extent as a matrixmaterial for phosphorescent dopants. The shorter the wavelengths of theradiation emitted from the dopant, the greater the energy differencebetween the T₁ level and the energetic ground state. This means that theuse of phosphorescence dopants which emit in the blue spectral range, onaccount of their high T₁ levels, is more difficult to be able tocoordinate with the T₁ levels of possible matrix materials than the useof phosphorescent dopants which emit in the red range.

The present device solves the problem of energy level matching throughthe combination of fluorescent substances (singlet emitters) andphosphorescent substances (triplet emitters) in different layers, thatis to say in different layers which can have different matrixsubstances. The same analogously also holds true for the energy level ofthe fluorescence dopant from which fluorescence takes place (S₁ level)and the corresponding S₁ level of the matrix, provided that the matrixdoes not itself function as an emitter.

If the embodiment of a device contains more than just one layer providedwith phosphorescent dopants, then the sequence of these layers must betaken into consideration. The layer with the phosphorescent dopanthaving the lowest T₁ level should be furthest away from the layer inwhich the excitons are formed, that is to say be the last one of thelayers with a phosphorescent substance through which triplet excitonspass. That is unless the energy transfer from the matrix material to thedopant, on account of the position of the energy levels of the excitedstates in said layer, is significantly poorer and less efficient than inthe other layers having other matrix-dopant combinations; in which casein turn said layer, in the layer sequence of the device, should liecloser again to the first charge carrier transporting layer, in whichthe excitons are formed, in order to obtain the best possible radiationyield.

One preferred embodiment comprises a second charge carrier transportinglayer comprising phosphorescent substances that emit in differentwavelengths. In this case, the second charge carrier transporting layercan be divided into partial regions, each of which comprises a differentphosphorescent substance. In this case, the partial region locatedclosest to the first charge carrier transporting layer comprises thephosphorescent substance which emits radiation having the shortestwavelength. The further partial regions comprise other phosphorescentsubstances in which the wavelength of the radiation emitted by themincreases with increasing distance from the first charge carriertransporting layer.

If the second charge carrier transporting layer comprises twophosphorescent substances, for example, then it can be divided into twopartial regions. A first partial region comprises, for example, aphosphorescent substance that emits in the green range, and a secondpartial region comprises a phosphorescent substance that emits in thered range. The first partial region is then arranged closer to the firstcharge carrier transporting layer than the second partial region.

A further preferred embodiment comprises at least two second chargecarrier transporting layers having different matrix materials. Thephosphorescent substances of these layers emit radiation havingdifferent wavelengths. In this case, the second charge carriertransporting layers are arranged in such a way that the wavelength ofthe radiation emitted by the phosphorescent substances increases withincreasing distance from the first charge carrier transporting layer.

Thus, one embodiment comprises, for example, two second charge carriertransporting layers, wherein one second charge carrier transportinglayer comprises a substance which is phosphorescent in the green rangeand the other second charge carrier transporting layer comprises asubstance which is phosphorescent in the red range. Said one secondcharge carrier transporting layer with the green emitter is thenarranged closer to the first charge carrier transporting layer than theother second charge carrier transporting layer with the red emitter.

A further aspect is also the desired hue which the device, e.g. theOLED, is intended to emit, that is to say the sum of the spectra of thefluorescent and phosphorescent substances. The proportion of theradiation which the individual dopants contribute to the overallspectrum is influenced by varying the sequence of individual layerscomprising the phosphorescent or fluorescent dopants. In this case, notonly the position but also the thickness of the respective layer plays apart, as does the density with which the dopant is introduced the matrixlayer. The closer the second charge carrier transporting layer havingthe phosphorescent substance as dopant, for example, is located to thefirst charge carrier transporting layer, the exciton forming layer, thebetter the energy is transferred from the matrix material to the dopant,the thicker the matrix material and the higher the proportion of thedopant in the matrix, then the greater the contribution of the emissionof said dopant to the overall spectrum of the OLED. In the case wherethe second charge carrier transporting layer was produced predominantlyor even exclusively from the phosphorescent substance, then the positionthereof in the layer sequence and its layer thickness are crucial forthat proportion of the total emission of the radiation emitting devicewhich is made up of the emission from said layer.

A further embodiment of an OLED according to the invention comprises atleast two charge carrier transporting layers, wherein the first chargecarrier transporting layer itself comprises a fluorescent material orwas doped with a fluorescent material. Fluorescence involves visibleradiation that is emitted as a result of relaxation from an excitedsinglet state to the ground state. The second charge carriertransporting layer contains a phosphorescent substance. Phosphorescenceinvolves visible radiation that is emitted as a result of relaxationfrom an excited triplet state to the ground state.

The second charge carrier transporting layer is substantially free ofcharge carriers having the first charge and transports only chargecarriers having the second charge. Therefore, recombination of electronsand holes does not take place in the second charge carrier transportinglayer and, consequently, nor are any excitons formed in said layer. Theconsequence of this is that the phosphorescent substance in said layercannot be excited by energy originating from excitons that were formedin said layer. This in turn means that the phosphorescent substance canonly be excited by energy that was transferred from another layer intothe layer in which the phosphorescent substance is situated. This energytransport can be effected for example by means of the so-called Dextertransfer mechanism or the Förster transfer mechanism. The Dextertransfer mechanism is an electron exchange mechanism via overlappingorbitals or wave functions between different molecules, e.g. afluorescent substance and the matrix material. In order to be able toexchange electrons, the molecules between which the electron exchange isto take place require appropriate redox potentials. In the case of theFörster transfer mechanism, a dipole-dipole interaction is responsiblefor the energy transfer. For this purpose, the two molecules must have aspectral overlap. According to the inventors' recognition, however, bymeans of the Förster mechanism the energy can only be transferred fromone singlet state to another singlet state, whereas by means of a Dextermechanism the energy can also be implemented from one triplet state toanother triplet state since here it is only necessary to comply with thelaw of conservation of spin.

On account of the combination according to the invention of fluorescentsubstances (singlet emitters) and phosphorescent substance (tripletemitters) in layers between which an energy transfer is possible, asignificantly higher radiation yield can be obtained than is possiblewith singlet emitters and triplet emitters which lie in layers betweenwhich no energy transfer takes place. The spatial separation of singletand triplet emitters additionally ensures that no radiationlessquenching processes are possible between the two systems. The spectralextension of the overall emission spectrum of the device is alsoadvantageous. On account of the conversion of the energy of the tripletexcitons that is otherwise emitted as thermal radiation into visibletriplet emission, an increased lifetime of the device results on accountof the reduction of the thermal radiation.

Substances suitable for producing a device according to the inventionare presented below. This enumeration should be regarded as mentioningpossible examples and not as an exhaustive list.

The following materials or combinations of materials are suitable forexample for the hole injection layer (HIL): A matrix plus F4-TCNQ(tetrafluoro-tetracyanoquinoline) or derivatives thereof and molybdenumoxides.

The following materials are suitable for example for the holetransporting layer (HTL):

-   1-TNATA (4,4′,4″-tris(N-(naphth-1-yl)-N-phenyl-amino)triphenylamine,    2-TNATA (4,4′,4″-tris(N-(naphth-2-yl)-N-phenyl-amino)triphenylamine,    MTDATA    (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine), aNPB    (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine), bNPD    (N,N′-bis(naphthalen-2-yl)-N,N′-bis(phenyl)benzidine), TPD    (N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine), spTAD    (2,2′,7,7′-diphenylamino-spiro-9,9′-bifluorene), Cu-PC    (phthalocyanine-copper complex) or other PC-metal complexes, TAPC    (1,1-bis-[(4-phenyl-)-bis-(4′,4″-methyl-phenyl)-amino]-cyclohexane).

The following materials are suitable, for example, for the electrontransporting layer (ETL):

-   Alq₃ (tris(8-hydroxyquinoline)aluminum, BAlq₂    (bis-[2-methyl-8-quinolato)-[4-phenylphenolato]-aluminum (III)),    BPhen (4,7-diphenyl-1,10-phenanthroline), BCP    (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), OXD7, OXD8, TPBi    (1,3,5-tris-(1-phenyl-1H-benzimidatol-2-yl)-benzene), TAZ    (3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)1-2,4-triazole), TAZ2    (3,5-diphenyl-4-naphth-1-yl-1,2,4-triazole), t-Bu-PBD    (2-(-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole), triazine    or triazine derivatives.

The following phosphorescent materials or combinations of materials aresuitable for example as phosphorescent substance:

-   FIr6, FPt1    ([2-(4′,6′-difluorophenyl)-pyridinato)-acetylacetonate[-platinum-II),    FIrpic    (bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)-iridium-III),    FIrN4, Irppy3 (fac-tris(2-phenyl-pyridyl)iridium complex),    Ir(ppy)₂acac, Ir(type)₃ (tris[2-(4-totyl)-pyridinato]-iridium(III)),    Ir(typ)₂acac, Ir(bt)₂acac, Ir(btp)₂acac    (bis[2-(2′-benzothienyl-pyridinato]-[acetyl-acetonato]-iridium(III)),    Ir(dbp)₂acac    (iridium(III)bis(dibenzo-[f,h]quinoxaline)(acetylacetonate)),    Ir(mdp)₂acac    (iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate)),    Ir(pq)₃, Ir(pq)₂acac, Ir(piq)₃, (CF3ppy)₂Ir(pic)m PtOEP    (platiniumoctaethylporphyrine).

The following materials or combinations of materials are suitable forexample for the layer or as matrix material in which both thefluorescent and the phosphorescent substance can be incorporated:

-   CBP (4,4′-bis(carbazol-9-yl)-2-2′dimethyl-biphenyl), TCTA    (4,4′,4″-tris(n-(naphth-2-yl)-N-phenyl-amino)triphenylamine), mCP,    TCP (1,3,5-tris-carcazol-9-yl-benzene), CPF, CDBP    (4,4′-bis(carbazol-9-yl)-2,2′-dimethyl-biphenyl), DPVBi    (4,4-bis(2,2-diphenyl-ethen-1-yl)-diphenyl), spiro-PVBi    (spiro-4,4′-bis(2,2-diphenyl-ethen-1-yl)-diphenyl), UGH1, UGH2,    UGH3, UGH4, CzSi, ADN (9,10-Di(2-naphthyl)anthracene), TBADN, MADN,    perylene, carbazole derivatives, fluorene derivatives.

The following materials or combinations of materials are suitable forexample as fluorescent substance:

-   DCM    (4-(dicyanomethylene)-2-methyl-6-(p-dimethylamino-styryl)4H-pyrane),    DCM2    (4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyrane),    DCJTB, rubrene(5,6,11,12-tetraphenyl-naphthacene), coumarin (C545T),    BCzVBi, BCzVb, TBSA    (9,10-bis[(2″,7″″-di-t-butyl)-9′,9″-spirobifluorenyl]anthracene),    DPAVBi, DPAVB, Zn complexes, Cu complexes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of one embodiment of a radiationemitting device.

FIG. 2 shows a schematic side view of a possible further embodiment of aradiation emitting device.

FIG. 3 shows a schematic side view of a possible further embodiment of aradiation emitting device.

FIG. 4 shows a schematic side view of a special embodiment of aradiation emitting device.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of one embodiment of the radiationemitting device. The latter comprises the five layers illustrated. Inthis case, the bottommost layer 9 is a substrate, and the succeedinglayer 1 is the anode. Layer 2 is a second charge carrier transportinglayer, a unipolar hole transporting layer, which cannot transportelectrons or blocks electrons. Arranged thereon is the first chargecarrier transporting layer 3, which transports both electrons and holesand is therefore ambipolar. The cathode is represented by layer 4. Thesecond charge carrier transporting layer 2 contains the phosphorescentsubstance P, and the first charge carrier transporting layer 3 containsthe fluorescent substance F. The electrons, illustrated schematically asencircled minus signs, are injected from the cathode 4 into the firstcharge carrier transporting layer 3, illustrated schematically by thearrow L1. From said layer they cannot diffuse further into the secondcharge carrier transporting layer 2 since the latter blocks or cannottransport electrons, illustrated schematically by the crossed out arrowL3. The holes, illustrated schematically as encircled plus signs, whichare injected from the anode 1 into the second charge carriertransporting layer 2, are transported by the second charge carriertransporting layer 2 into the first charge carrier transporting layer 3,illustrated schematically by the arrow L2. The excitons, illustratedschematically as an asterisk, formed by recombination of electrons andholes in the first charge carrier transporting layer 3 can diffuse intothe second charge carrier transporting layer 2, or the energy,illustrated schematically by the arrow E, can be transferred by means ofenergy transfer mechanisms into the second charge carrier transportinglayer 2 and excite the phosphorescent substance P to phosphorescencethere. Since the second charge carrier transporting layer 2 is largelyfree of electrons, recombination of electrons and holes cannot takeplace and, consequently, nor can any excitons be formed in said layer.The fluorescent substance F situated in the first charge carriertransporting layer 3 is likewise excited by the energy of the excitons,which are formed in this layer, by means of energy transfer mechanisms.Consequently, different electroluminescent substances (F and P) arrangedin different layers (3 and 2) are electronically excited by energy ofexcitons, whereas the energy originates from the first charge carriertransporting layer 3.

FIG. 2 shows a schematic side view of a further embodiment of theradiation emitting device. The latter comprises six layers illustrated.In this case, the bottommost layer 9 is a substrate, and the succeedinglayer 1 is the anode. Layer 2 is the second charge carrier transportinglayer, a hole transporting layer. The layer 5 represents a first chargecarrier blocking layer, here an electron blocking layer. Arrangedthereon is the first charge carrier transporting layer 3, a layer whichtransports both electrons and holes. The cathode is represented by layer4. The second charge carrier transporting layer 2 contains thephosphorescent substance P and the first charge carrier transportinglayer 3 contains the fluorescent substance F. The electrons, representedschematically as encircled minus signs, are injected from the cathode 4into the first charge carrier transporting layer 3. From said layer theycannot diffuse further into the layer 5 since the latter blockselectrons, illustrated schematically by the crossed out arrow L3. Theholes, illustrated schematically as encircled plus signs, which areinjected from the cathode into the second charge carrier transportinglayer, are transported from the second charge carrier transporting layer2 through the layer 5 into the first charge carrier transporting layer3, illustrated schematically by the arrow L2. The excitons, illustratedschematically as an asterisk, formed by recombination of electrons andholes in the first charge carrier transporting layer 3 can diffusethrough the layer 5 into the second charge carrier transporting layer 2,or the energy, illustrated schematically by the arrow E, can betransferred by means of energy transfer mechanisms into the secondcharge carrier transporting layer 2 and excite the phosphorescentsubstance P to phosphorescence there. The layer 5 blocks the chargecarriers having the first charge, here the electrons, such that thelayer 2 is largely free of electrons. Thus, recombination of electronsand holes cannot take place and, consequently, excitons cannot be formedin the second charge carrier transporting layer 2. The spatialseparation between the layer 2 containing the phosphorescent substance Pand the first charge carrier transporting layer 3 containing thefluorescent substance F is even larger in the embodiment illustrated inFIG. 2 than in the embodiment illustrated in FIG. 1, in which the layers2 and 3 are directly adjacent.

FIG. 3 shows a schematic side view of a further embodiment of theradiation emitting device. The latter comprises the six layersillustrated. In this case, the bottommost layer 9 is a substrate, andthe succeeding layer 1 is the anode. Layer 6 represents an excitonblocking layer. Layer 2 is the second charge carrier transporting layer,a hole transporting layer, which does not transport electrons ortransports them only to a subordinate extent. Arranged on the holetransporting layer 2 is the first charge carrier transporting layer 3,which transports both electrons and holes. The cathode is represented bylayer 4. The second charge carrier transporting layer 2 contains thephosphorescent substance P, and the first charge carrier transportinglayer 3 contains the fluorescent substance F. The electrons, illustratedschematically as encircled minus signs, are injected from the cathode 4into the first charge carrier transporting layer 3, illustratedschematically by the arrow L1. From said layer they cannot diffusefurther into the second charge carrier transporting layer 2 since thelatter blocks or cannot transport electrons, illustrated schematicallyby the crossed out arrow L3. The holes, illustrated schematically asencircled plus signs, which are injected from the anode 1 into theexciton blocking layer 6, are transported through the second chargecarrier transporting layer 2 into the first charge carrier transportinglayer 3, illustrated schematically by the arrow L2. The excitons,illustrated schematically as an asterisk, formed by recombination ofelectrons and holes in the first charge carrier transporting layer 3 candiffuse into the second charge carrier transporting layer 2, or theenergy, illustrated schematically by the arrow E, can be transferred bymeans of energy transfer mechanisms into the second charge carriertransporting layer 2 and excite the phosphorescent substance P tophosphorescence there. Since the second charge carrier transportinglayer 2 is largely free of electrons, recombination cannot take placeand, consequently, excitons cannot be formed in said layer. Through thelayer 6, the excitons coming from the first charge carrier transportinglayer 3 cannot diffuse through the second charge carrier transportinglayer 2 into succeeding layers, for example in the direction of theanode, since layer 6 blocks excitons. The radiationless quenching of theexcitons at the anode is thus prevented or reduced. The energy of theexcitons, primarily of the triplet excitons, can thus largely be takenup by the phosphorescence substance P of the second charge carriertransporting layer 2, which increases the radiation yield of the device.

FIG. 4 shows a schematic side view of a further embodiment of theradiation emitting device, an OLED. The six layers illustrated representthe following constituents of the invention. Layer 9 represents asubstrate, on which the further layers are applied. Layer 10 is atransparent anode. 20 is a hole inducing layer (HIL), which lowers theenergy barrier for hole transfer from the anode into the holetransporting layer. 30 is a hole transporting layer (HTL) containing aphosphorescent substance P. 40 is a layer which transports bothelectrons and holes and which comprises a fluorescent substance F. Thelayer 50 represents the cathode.

One preferred embodiment is an OLED having the construction asillustrated schematically in FIG. 4. The OLED is constructed from thefollowing layers: ITO/PEDOT/HTL/LEP/cathode. In this case, ITO (indiumtin oxide) is the transparent anode 10, PEDOT is the hole inducing layer20, and HTL is a hole transporting layer 30. LEP represents a lightemitting layer 40, wherein the layer 40 emits light in the form offluorescence. The cathode composed of aluminum (200 nm) with a thinlayer of CsF (1 nm) is represented by the layer 50. In this exemplaryembodiment, the LEP 40 is a polymer which fluoresces in the blue range.The HTL 30 contains a phosphorescent substance P which phosphoresces inthe red range, that is to say likewise emits light. The material of theHTL 30 is chosen such that holes are transported but electrons are not.That is to say that a meeting of electrons and holes and, consequently,a recombination are possible only in the LEP 40. One part of the energyreleased by the recombination of electrons and holes then excites thepolymer situated in said layer into the singlet state, from which saidpolymer then emits with emission of light in the blue wavelength range.Another part of the energy, primarily the triplet excitons, istransported by means of energy transfer processes into the HTL 30, inwhich the phosphorescent substance P is situated. The latter is excitedto a triplet level by the energy transferred from the LEP 40, from whichlevel the phosphorescent substance P then relaxes with emission ofradiation in the visible range.

The spectrum of such an OLED also has, besides the bands of fluorescentlight, a spectrum generally extended to longer wavelengths, as a resultof the additional phosphorescence emission. By means of a correspondingchoice of materials, such an OLED can generate white light.

The invention is not restricted by the description on the basis of theexemplary embodiments. Rather, the invention encompasses any new featureand any combination of features, which in particular comprises anycombination of features in the patent claims, even if this feature orthis combination itself is not explicitly explained in the patent claimsor exemplary embodiments.

1. A radiation emitting device comprising: a first electrode, whichemits first charge carriers having a first charge during operation; afirst charge carrier transporting layer, which comprises a fluorescentsubstance and is arranged on the first electrode; a second chargecarrier transporting layer, which contains a phosphorescent substanceand is arranged on the first charge carrier transporting layer; and asecond electrode, which emits second charge carriers having a secondcharge during operation and is arranged on the second charge carriertransporting layer; wherein during operation the second charge carriertransporting layer is largely free of first charge carriers emitted bythe first electrode.
 2. The radiation emitting device as claimed inclaim 1, wherein the second charge carrier transporting layer isunipolar and can transport only second charge carriers, or blocks firstcharge carriers.
 3. The radiation emitting device as claimed in claim 1,wherein a unipolar charge carrier transporting layer, which blocks thecharge carriers having the first charge or can transport only chargecarriers having the second charge, is arranged between the first chargecarrier transporting layer and the second charge carrier transportinglayer.
 4. The radiation emitting device as claimed in claim 1, embodiedas an organic light emitting diode, wherein the first charge carriertransporting layer (3), the second charge carrier transporting layer(2), the phosphorescent substance or the fluorescent substance compriseorganic materials.
 5. The radiation emitting device as claimed in claim1, wherein a recombination of charge carriers having the first chargeand charge carriers having the second charge can result in the formationof excitons as excited electronic states, which comprise singlet andtriplet excitons.
 6. The radiation emitting device as claimed claim 1,wherein a recombination of charge carriers having the first charge andcharge carriers having the second charge takes place only in the firstcharge carrier transporting layer.
 7. The radiation emitting device asclaimed in claim 5, wherein the excitons formed in the first chargecarrier transporting layer comprise triplet excitons, and wherein thephosphorescent substance emits phosphorescent radiation upon excitationby the energy of the triplet excitons formed in the first charge carriertransporting layer.
 8. The radiation emitting device as claimed in claim1, wherein the first electrode is an anode, the first charge carriertransporting layer is a hole transporting and electron transportinglayer, the second charge carrier transporting layer is an electrontransporting layer, and the second electrode is a cathode.
 9. Theradiation emitting device as claimed in claim 8, wherein the secondcharge carrier transporting layer is a hole blocking, or exclusivelyelectron transporting layer.
 10. The radiation emitting device asclaimed in claim 1, wherein the first electrode is a cathode, whereinthe first charge carrier transporting layer is a hole transporting andelectron transporting layer, wherein the second charge carriertransporting layer is a hole transporting layer, and wherein the secondelectrode is an anode.
 11. The radiation emitting device as claimed inclaim 10, wherein the second charge carrier transporting layer is anelectron blocking or exclusively hole transporting layer.
 12. Theradiation emitting device as claimed in claim 1, wherein an excitonblocking layer is situated between the second charge carriertransporting layer and the second electrode.
 13. The radiation emittingdevice as claimed in claim 1, wherein the fluorescent substancecomprises an organic material.
 14. The radiation emitting device asclaimed in claim 1, wherein the fluorescent substance is present as adopant in a charge carrier transporting matrix material.
 15. Theradiation emitting device as claimed in claim 1, wherein thephosphorescent substance comprises an organic material.
 16. Theradiation emitting device as claimed in claim 1, wherein thephosphorescent substance is present as a dopant in a charge carriertransporting matrix material.
 17. The radiation emitting device asclaimed in claim 16, wherein at least two different phosphorescentsubstances are situated as dopants in the second charge carriertransporting layer.
 18. The radiation emitting device as claimed inclaim 17, wherein the phosphorescent substances emit with differentwavelengths.
 19. The radiation emitting device as claimed in claim 18,wherein the second charge carrier transporting layer has partial regionswhich each comprise a substance that is phosphorescent at differentwavelengths, and the wavelength of the emitted radiation of thephosphorescent substance increases with greater distance from the firstcharge carrier transporting layer.
 20. The radiation emitting device asclaimed in claim 1, wherein the HOMO/LUMO levels of the second chargecarrier transporting layer are coordinated with the HOMO/LUMO levels ofthe first charge carrier transporting layer such that no charge carriershaving the first charge can pass into the second charge carriertransporting layer.
 21. The radiation emitting device as claimed inclaim 1, wherein the HOMO/LUMO levels of the second charge carriertransporting layer are coordinated with the HOMO/LUMO levels of thefirst charge carrier transporting layer such that triplet excitons canpass from the first into the second charge carrier transporting layer.22. The radiation emitting device as claimed in claim 1, wherein thethickness of the second charge carrier transporting layer is coordinatedwith the diffusion length of the triplet excitons, such that the tripletexcitons can diffuse through the entire second charge carriertransporting layer.
 23. The radiation emitting device as claimed inclaim 1, wherein at least two second charge carrier transporting layersare present and the total thickness of all the second charge carriertransporting layers is less than or equal to the diffusion length of thetriplet excitons.
 24. The radiation emitting device as claimed in claim1, wherein each second charge carrier transporting layer comprises aphosphorescent substance that emits at a different wavelength.
 25. Theradiation emitting device as claimed in claim 24, wherein the secondcharge carrier transporting layers are arranged such that the wavelengthof the emitted radiation increases with greater distance from the firstcharge carrier transporting layer.
 26. The radiation emitting device asclaimed in claim 1, wherein the HOMO/LUMO levels of all the secondcharge carrier transporting layers are coordinated with one another suchthat an exciton transfer through all the charge carrier transportinglayers can be effected from the first charge carrier transporting layer.27. The radiation emitting device as claimed in claim 1, wherein acharge carrier transport path for transporting the first and secondcharge carriers during operation runs through the device and the chargecarrier transport path for the first charge carriers is restricted tothat region of the device which is delimited by the first electrode andthe first charge carrier transporting layer and the charge carriertransport path of the second charge carriers runs at least through thesecond charge carrier transporting layer and the first charge carriertransporting layer.